If you don't want to wait for more hextech chests and keys, you can buy a mystery shard.
The shard will be for a random champion.
If it's Karma, then you can just use the shard (this is the cheapest possibility)
If it's a 450 shard, then you're best off saving 3x 450 shards for a reroll into a permanent.
If you get a permanent Karma through this (although that's only possible if you have all champions unlocked) then you can use the Karma permanent to unlock the token (second cheapest possibility)
If you got a permanent for a champion you don't want, then you can disenchant that permanent for essence.
If the shard was for a 1350 or greater champion then, assuming you don't want to use the shard to unlock the champion or reroll for random permanent champions to unlock, you can disenchant it for essence to use to unlock your token.
Essence is the least efficient way to unlock a token, but getting the correct shard or permanent is unreasonably rare.